Modern radar and communications systems must be designed to cope with directional interference, whether intentional or not, in order to satisfy the needs of the users. This requirement has led to increasing interest in adaptive antenna array techniques in recent years. "Adaptive Arrays and Sidelobe Cancellers; A Perspective" by D. J. Chapman, Microwave Journal, August-1977, presents a general view of this field of work. Another source of information on this subject is "Adaptive Arrays--An Introduction" by William F Gabriel, Proceedings of the IEEE, Vol. 64, No. 2, February 1976.
Another technique which has been developed is that of spread spectrum frequency hopping, such as is being used in the system known as JTIDS (Joint Tactical Information Distribution System) as described in "JTIDS: An Update" by Edward G. Smith, American Institute of Aeronautics and Astronautics, 1977. This technique is also discussed in "Spread Spectrum Systems" by Dixon, Wiley Interscience, 1966, p. 26 et seq.